During the last year we have shown the following: 1) About 5% of the nuclear newly-synthesized polyoma RNA is complementary to the E-strands and 95% is complementary to the L-strands and this proportion is independent of the labeling time. These results indicate that in addition to post-transcriptional controls and additional mechanism determines the frequencies of transcription from the E and L strands. 2) We have investigated the transcription of a cloned substituted SV40 genome of defined structure and found that the inserted DNA sequences, both of the nonreiterated and reiterated type are abundantly transcribed in intact cells coinfected with the variant and wild type virus. 3) We investigated the "cap" structures of SV40 mRNAs and found that 19S viral mRNA contains approximately equal proportion of m7GpppAm and m7Gpppm6Am; while 16S mRNA contains only m7Gpppm6Am. N6methyl adenosine is located internally within the RNA chains of both the 19S and 16S species. 4) We have determined the initiation site for late transcription at coordinate 0.67 plus or minus 0.01 on the physical map of SV40 DNA. 5) The highlights of our work during the last year has been the discovery that the leader sequences, those found at the 5' ends of the main bodies of SV40 late mRNA, and the adjacent coding sequences, are not transcribed from a continuous segment of the viral DNA.